<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>webgl缓冲区对象的使用_05</title>
  <script src="lib/webgl-debug.js"></script>
  <script src="lib/webgl-utils.js"></script>
  <script src="lib/cuon-utils.js"></script>
  <script src="lib/cuon-matrix.js"></script>
</head>

<body onload="main()">
  <canvas id="canvas" width="400" height="490"></canvas>
</body>
<script>
  // 顶点着色器伪代码
  var vShder_source = `
  attribute vec4 a_Position;
void main(){
gl_Position =a_Position; //设置坐标
gl_PointSize =10.0; //设置尺寸
}
`
  //片元着色器伪代码
  var fShder_source = `
void main(){
gl_FragColor = vec4(1,0.0,0.0,1.0); //设置颜色
}
`
  function main() {
    var canvas = document.getElementById('canvas');
    //  获取上下文对象
    var gl = getWebGLContext(canvas);
    //  初始化着色器
    if (!initShaders(gl, vShder_source, fShder_source)) {
      return;
    }
    // 指定一个覆盖canvas的颜色
    gl.clearColor(0.0, 0, 0, 1.0);
    // 清除指定的缓冲区
    gl.clear(gl.COLOR_BUFFER_BIT);
    var n = initVertexBuffers(gl);
    //将三个点绘制出来
    gl.drawArrays(gl.POINTS, 0, n);
  }

  //初始化顶点缓存区
  function initVertexBuffers(gl) {

    var vertices = new Float32Array([0.0, 0.5, -0.5, -0.5, 0.5, -0.5]);
    var n = 3;

    //创建缓存区
    var vertexBuffer = gl.createBuffer();
    if (!vertices) {
      console.log("创建缓存区失败");
      return -1;
    }
    // 绑定缓冲区到目标
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
    // 项缓冲区写入数据
    gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);
    //获取 attribute 变量的储存位置
    var a_Position = gl.getAttribLocation(gl.program, "a_Position");
    if (a_Position < 0) {
      return -1;
    }
    //将缓冲区对象分配给 a_Position变量
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
    // 链接a_Position 变量与分配给他的缓冲区对象；
    gl.enableVertexAttribArray(a_Position);
    return n;
  }

</script>

</html>